What are the privacy problems of using a Ledger to manage cryptocurrencies?

Hello, what privacy problems are there in using a Ledger? Do Ledger know how many crypto I have?

Using a Ledger or any hardware wallet is not a privacy concern. Using the Ledger Live app with your Ledger is though, because not only can Ledger see your transactions (as anyone can on the Blockchain) but they can trivially tie that to your IP address and other device info, because they store all that metadata on their nodes.

If you are concerned about privacy with Ledger you should use a local wallet app which works with Ledger instead of Ledger Live, which will depend on the cryptocurrencies you use of course (for example: MetaMask for Ethereum, Electrum for Bitcoin, etc.).

All that being said, virtually all cryptocurrencies besides Monero will record every transaction you make on the permanent blockchain which anyone can view, so I would not use them for transactions requiring privacy in the first place.